The electrolysis process for producing aluminum is well-known. The process uses an electrolysis cell having a number of anodes and cathodes. By passing a current through a raw material such as alumina located between an anode and a cathode, molten aluminum is produced. Once molten, essentially pure aluminum metal is withdrawn. In such cells, the cathodes are usually fixed and cooperate with a number of movable carbon-based anodes which are consumed during the electrolysis process. Carbon is used because it can be consumed without adding impurities to the product aluminum. To maintain optimum conditions, i.e., minimize power consumption, the spacing or gap between the cathode and anode should be maintained at an essentially constant distance. Thus, as the carbon anode is consumed, the gap increases and the anode must be lowered to maintain the optimum gap.
For ease in illustration, a single anode/cathode pair will be discussed. Typically, the anode has an upwardly extending rod which is attached to a movable anode beam. The anode is thus suspended from the beam which controls the movement of the anode. The anode beam is in turn engaged by a mechanism for raising or lowering the anode beam. Thus, the anode beam is lowered in an amount corresponding to the consumption of the carbon anode. When the anode beam reaches its lowest point, each anode is individually attached to an auxiliary cross arm, which is mounted on an end-side supporting frame. The locks holding the anodes to the beam are then released, and the anode beam is raised to its highest position. The anodes are then reattached to the beam for further lowering. Of course, such a procedure requires a halt to production, and, to minimize these stoppages, a long anode rod and a large difference in anode beam travel.
Following this method, the difference between the highest and lowest positions of the anode beam is fairly large, on the order of 250-400 mm (25-40 cm), resulting in a large current path with a consequent high power loss. Another result of the large amount in the beam travel is a fluctuation in the magnetic field around the cell which may disrupt cell efficiency. Also, it is quite time consuming to detach and reattach the anodes to the anode beam.
Ideally, the distance between poles (distance from the underside of the anode to the cathode) is the same for all anode carbons, and the electrolysis current distributes itself uniformly over all anode carbons. In operation, however, deviations from the ideal case occur, as each anode is consumed at a varying rate, and this deviation must- be corrected to prevent the scatter of the current distributions over the anodes from exceeding a certain limit with a loss in efficiency. To correct this deviation, the pole spacing of individual anode carbons must be increased or decreased to account for the deviations in consumption.
Another problem with electrolysis cells involves changes in efficiency due to changes in the electrolyte. Upon aluminum oxide depletion from the electrolyte, the so-called "anode effect" occurs, where, during the anode effect, the furnace voltage demand increases from about 4 V to about 30 V. This voltage rise causes greater energy consumption and therefore must be eliminated quickly. To eliminate the anode effect, aluminum oxide is added to the electrolyte, increasing the bath volume. It is common practice that all anode carbons are moved up from the metal bath to prevent local shortcircuiting during the addition. During return of the anodes to there optimum spacing, the downward movement of the anode carbons may cause overflow of the melt due to displacement of the metal bath.